The present invention relates to a mask x-ray lithography, and more particularly relates to improvement in x-ray permeability and mechanical properties of a mask made up of a substrate and at least one pattern layer and used for x-ray lithography.
In a typical construction of a mask used for this purpose, at least one pattern layer is deposited on a substrate in a prescribed design. Conventionally, the pattern layer is in the thickness of 0.1 to 0.5 .mu.m and made of a metal of high soft x-ray absorption such as Ag and W. (It is said that the wave length of the soft x-ray is 4 through 14.ANG.). Whereas the substrates is in the thickness of several .mu.m and made of a ceramic of relatively high soft x-ray permeability such as BN, Si.sub.3 N.sub.4, SiC and SiO.sub.2.
The conventional mask of the above-described composition is inevitably accompanied with several fatal demerits. In the first place, the x-ray permeability of the ceramic used for the substrate is not high enough to allow substantially full permeation of the soft x-ray radiated and negligible part of the soft x-ray radiated is absorbed by the substrate. As a result, the ratio of permeation between the permeable region (the substrate only) and the impermeable region (the substrate covered by the pattern layer) is not high enough to provide a clear contrast. In order to raise this ratio of permeation, it is thinkable to decrease the thickness of the substrate. Such reduction in thickness of the substrate inevitably would lead to lowering in mechanical strength of the mask. In addition, undersirable warp would be developed in the substrate by x-ray radiation.
Since the materials for the substrate and the pattern layer are both given in the form of compounds, the materials tend to change their physical by long period x-ray radiation. Further, the substrate ceramic and the pattern layer made of metal are significantly different in degree of thermal expansion and, as a consequence, they exhibit poor coordination in physical behavior when heated.